RESEARCH ARTICLE Population Response of Giant Gal´ apagos Tortoises to Feral Goat Removal Cruz M´ arquez, 1 James P. Gibbs, 1,2,3 Victor Carri´ on, 4 Sixto Naranjo, 4 and Alizon Llerena 1 Abstract Efforts to eradicate nonnative mammals to restore oceanic island ecosystems have become increasingly successful but parallel tracking of response by native species for which control efforts are intended to benefit has been rare. A major campaign to eradicate nonnative goats and burros was initiated in 1995 on Alcedo Volcano in the Gal´ apagos Archipelago that ultimately removed 62,868 goats and eliminated them by 2006. Planners of the eradication pro- gram had the foresight to invest in intensive monitoring of the status of the volcano’s giant tortoise (Chelonoidis nigra ) population whose welfare was a primary motivation for the eradication effort. Monitoring revealed an increase in the proportion of juveniles among all tortoises as well as increased growth rates of individual tortoises on Alcedo Volcano from earlier to later phases of the eradication cam- paign. Over the same time frame in a control population on nearby Santa Cruz Island (where goats and donkeys were not removed) juvenile fraction and individual growth rates remained unchanged. Although goat removal coin- cided with occurrence of a rare climatic event that simul- taneously boosted forage availability for tortoises, failure to observe a comparable improvement in the control pop- ulation implies that removal of goats and burros was the primary causative factor of improving population status of tortoises on Alcedo Volcano. Key words: eradication, Gal´ apagos Islands, giant tortoise, goats, invasive species, population restoration. Introduction Introduction of mammals to islands has lead to destruction of endemic species and degradation of ecosystems around the globe (Towns et al. 2006). In response, nonnative mammal eradication efforts have become widespread (Donlan & Wilcox 2008). Integrating monitoring of the species targeted for erad- ication within an adaptive management framework underlies much of the recent and striking upswing of success of eradi- cation operations world-wide (Lavoie et al. 2007). This said, parallel tracking of population- and community-level response by the endemic species for which control efforts are often intended to benefit is rare (Courchamp et al. 2003). Lack of evaluation of population response by endemic species to non- native species control programs hinders development of sup- port for further eradication efforts to restore island ecosystems. On the Gal´ apagos Archipelago, feral goats (Capra hircus ) and donkeys (Equus asinus ) (Fig. 1a & 1f) cause major damage to the vegetation and fauna (Hamann 1993). Alcedo Volcano on Isabela Island is an example. Goats first arrived 1 Charles Darwin Foundation, Puerto Ayora, Gal´ apagos, Ecuador 2 Department of Environmental and Forest Biology, State University of New York College of Environmental Science and Forestry, Syracuse, NY 13210, U.S.A. 3 Address correspondence to J. P. Gibbs, email jpgibbs@esf.edu 4 Program for Protection and Conservation of Terrestrial Ecosystems, Gal´ apagos National Park Service, Puerto Ayora, Gal´ apagos, Ecuador  2012 Society for Ecological Restoration doi: 10.1111/j.1526-100X.2012.00891.x from the adjacent volcano of Sierra Negra to the south in the late 1960s and were reported in the early 1980s in such restricted areas and small numbers as to be considered not to represent a threat to the biota (Duffy 1981). By the mid 1990s, however, the goat population had increased from 80 to 100,000 animals, causing severe damage to the vegetation (Freire 1992). Heightened concern initially expressed by MacFarland (1991) and others about competition for food between goats and endemic giant tortoises (Chelonoidis nigra vandenburghi ) on Alcedo gave rise to concerted control efforts by the Gal´ apagos National Park Service under the aegis of Project Isabela in 1995, which lead to removal of 62,868 individuals at a cost of US$4.1 million and eradication by 2006 (Carrion et al. 2011). The architects of Project Isabela invested in monitoring not only progress on goat and donkey removal (Carrion et al. 2011) but also endemic species’ responses to invasive mammal control efforts (e.g. birds: Donlan et al. 2007 and invertebrates: Desender et al. 1999). Herein we describe the population response by giant tortoises on Alcedo Volcano (Fig. 1b) whose welfare was one of the primary motives for Project Isabela. Giant tortoises are an iconic animal for Gal´ apagos and also an important “ecosystem engineer” there (Gibbs et al. 2008, 2009; Blake et al. in press). Goats primarily and donkeys secondarily are considered major competitors for food with tortoises as well as causing other forms of habitat degradation, such as removing shade trees, denuding shrubs of the foliage upon which mist condenses and drips from to the ground Restoration Ecology 1